The class of analog circuits conventionally referred to as current sources typically includes current sources, sinks and current mirror amplifiers. Current sources are most commonly used to provide a reference current flow that serves as the basis for current driving other analog circuits.
In a simple current source of conventional design, a resistor is used to set the reference current level through one leg of a current mirror configured pair of transistors. While the appeal of such a simple circuit is obvious, and indeed, the circuit is more than adequate as a current source in many applications, the presence of the resistor essentially precludes its possible operation as a precision current source. Although the distinction between precision and ordinary current source designs is rather empirical, a precision current source is generally regarded as one whose reference current changes by less the 5000 parts per million (ppm) per degree centigrade over its specified operating temperature range. Current sources utilizing resistors typically fail to qualify as precision current sources due to the substantially temperature dependent value of their resistors. The value of a P-type resistor fabricated in a monolithic substrate will often vary by one or more percent per degree centigrade.
A somewhat related drawback to utilizing resistor-based current source designs, particularly in monolithically fabricated analog circuits, is the wide variance in resistive values realized as a result of normal fabrication process variation. Often there is a desire to be able to produce analog circuits of common design with closely matched operating characteristics. Expected, but difficult to eliminate, parameter fluctuations between processing runs of a monolithic analog circuit typically show up as significant variations in the value of the resistors formed. Consequently, the reference current-levels between circuits of identical design or both will vary unpredictably when separately fabricated in different processing runs or on different substrates, or both.
An alternative to current sources employing resistors is achieved with current source designs utilizing a series connected field effect transistor (FET) and a Zener diode voltage reference. The FET is placed in the primary leg of a current mirror and the Zener diode placed so as to be reversed biased in the second leg of the current leg. The gate of the FET is coupled to the base of the Zener diode. Consequently, the break-over voltage threshold of the Zener diode and the particular selected gate/source voltage-to-channel current relationship of the FET functions to constrain the level of current flow through the primary current mirror leg.
Zener diode design current sources have the advantage of not employing a true resistor. The temperature dependent characteristics of the resistively operated FET are such that precision operation of the current source is readily obtainable. However, the break-over voltage of the Zener diode is itself fairly temperature sensitive and highly dependent on fabrication parameters. Further, the required usage of a Zener diode alone places a substantial practical limitation on the usage of Zener diode type current sources in circuits of monolithic implementation. Zener diodes are difficult to fabricate with break-over voltages of less than 6.2 volts on a monolithic substrate in common with other active devices due to the very high impurity doping densities required. Consequently, regulated operation of the current source is generally not possible at source voltages of less than about 6 volts.